Thanks. So, how often does that typically happen? I know it's dependent on one's driving style, but at least in your case?

I'm trying to decide whether I should go ahead and buy a power bleeder. Or whether I can just flush every 6 track days.

It's a difficult question to answer. Typically I would get air in the calipers after 6 hours track time, but sometimes sooner. That's about 2 track days.
However, you need to feel if there's any air in the system.